All subsea vehicles and most subsea equipment require the use of a flotation system to make the vehicle or equipment either neutrally or positively buoyant. Typically, a castable material called syntactic foam is used for this purpose. This is especially true of subsea vehicles, such as Remotely Operated Vehicles (ROV's), and production oil and gas riser pipes (the piping that conducts oil and/or natural gas from the sea floor to a floating production platform at the surface of the ocean).
Syntactic foam is a mixture of epoxy or other suitable resin with hollow microspheres and sometimes "macrospheres" which typically are made of glass mixed evenly throughout the resin. "Macrospheres" are larger than microspheres, with sizes ranging up to about 3 inches in diameter. The syntactic foam is cast and cured to form a block. Since the resins are liquid at room temperature, the foam can be cast into very complex shapes.
The buoyancy efficiency of syntactic foam is defined as dry weight divided by the weight of a comparable volume of sea water. The smaller the buoyancy efficiency number, the more efficient the buoyancy of the foam. At a rated depth of 3000 meters in the ocean, sufficient buoyancy can be provided if the foam density is roughly half the density of water (0.5 g per cc or 32 pounds per cubic foot). At deeper depths, foam having significantly higher density is required.
This means that--in deeper water--considerably more foam is required to provide the same amount of buoyancy. For an ROV that will operate at 3000 to 6000 meters ocean water depth (10,000 to 20,000 feet), the amount or size of the block of syntactic foam required to provide a desired amount of buoyancy can become a significant problem. At a design depth of 6000 meters, a typical Work Class ROV would require a foam block nearly twice as large as the foam block that would be required at 3000 meters.
In addition to the problem of size, syntactic foam also is relatively expensive and lighter weight syntactic foams with greater buoyancy efficiency are subject to crushing at the pressures encountered in deep water. Syntactic foams are needed which are less expensive, which have increased buoyancy efficiency, and which have greater resistance to crushing in deep water.